Volatile chemicals can form expansive toxic gas clouds after an accidental or deliberate large-scale release. The emerging toxic clouds may be invisible to the optical spectrum of the bare eye, but they are generally detectable using suitable standoff or point detectors. Standoff detectors are particularly suited for monitoring a large area within their line of sight, whereas remotely controlled point detectors may be used to survey specific areas of strategic interest. A favorable spatial and temporal detection resolution is usually achieved using standoff Fourier Transform Infrared (FTIR) spectrometers. To obtain a proper spatial resolution beyond a mere imaging view, at least two imaging systems must operate concurrently with an adequate opening angle concerning the distance of reconnaissance. During a field trial in Umeå, Sweden, we utilized an appropriate setup for standoff tomography to detect and identify comparatively small-scale chemical releases of gaseous substances and evaporating aerosols. We reached high resolutions in space and time at a standoff distance of over a kilometer. Thus, we have shown that a targeted early warning and short response times for emerging threats are possible while operators remain at a safe location. Additionally, the field trial revealed the significant influence of the properties and concentration of the deployed chemicals, wind shear, and turbulence on the detection result. In support of spatially and temporally resolved standoff detection, targeted drones carrying fast and sensitive point detectors, such as ion mobility spectrometers, may be used as an orthogonal technique to independently confirm identification.
Most reported measurement efforts for visualizing gaseous exposure signatures aim to detect and analyze continuous releases of volatile chemicals. Recently, we became particularly interested in characterizing short-time explosive releases of chemical substances. To perform such experiments, we pursued the construction of a suitable device that generates appropriate short-time events in a reproducible manner. This device, which we refer to as an aerosol bomb, allows the controlled release of liquids from 10 to 200 mL within a timeframe of one to two seconds after being pressurized up to 80 bar. Furthermore, different spray profiles and, thus, different cloud shapes can be created using customized spray nozzles. These short-time chemical exposures, however, proved challenging to visualize by video recordings as dilution and volatilization led to the rapid disappearance of visible cloud shapes. Therefore, we utilized a dual setup of passive infrared (IR) Focal Plane Arrays to detect and identify these lower concentrations of chemicals. In preceding studies, we have already shown the application of an IR focal plane array detector for hyperspectral recording and analysis of measurement fields of various sizes with 128 x 128 pixels in a time grid of two seconds. After connecting two hyperspectral imaging measurement systems into a combined dual setup, we conducted a three-dimensional (3-D) characterization of short-time chemical exposures, whereby 3-D imaging is realized by intersecting beams of IR waves.
With the purpose of validating dispersion models, ammonia (NH3) releases were performed in September 2018 and a network consisting of NH3 detectors and temperature sensors were positioned in a grid in front of the source. In addition, the test grid was also monitored by a focal plane array imaging system based on a LWIR detector, which was positioned at a safe standoff distance of 1 km. With this setup, it was possible to monitor the release and the development of the generated cloud during the dissemination, as well as monitoring surrounding areas for risk assessment purposes during and after each challenge. As the observation was performed in near real time (approximately 0.5 Hz frame rate for the measurement, data transfer, Fourier transform and analysis), it was possible to give immediate feedback to the release team and test control personnel. Of special interest are background concentrations below the detection limit, as once these are achieved this indicates whether an area is safe and/or when additional challenges/disseminations can occur.
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